Systems for dispensing fuel into vehicles, e.g. at gasoline stations, typically employ a coaxial hose, which, in so-called inverted arrangement, has an outer conduit for delivery of fuel to the vehicle and an inner conduit for recovery of fuel vapors displaced from the vehicle tank. Recent field testing by the California Air Resources Board (CARB) of vacuum assist vapor recovery systems for use in fuel dispensing systems uncovered a problem with the inverted coaxial hose. They found that the internal vapor hose can kink during normal operation, causing substantial blockage of the vapor return pathway. Once kinked, these hoses tend to remain in a tube-collapsed condition, thus permanently reducing the cross sectional area of the vapor flow path.
Vacuum assist systems which employ a variable speed vane pump, or other flow control device mounted in the dispenser, use the electronic pulses related to the liquid gasoline flow rate to control the vane pump speed or the orifice size of a variable orifice restrictor. Using this technique, the volumetric rate of vapor recovery is maintained in proportion to the rate of liquid gasoline delivery.
A kink in the vapor hose will act to restrict vapor flow, thus changing the absolute pressure on the inlet side of the vapor flow control device. The reduction in pressure results in a reduction in throughput for both the vane pump and the variable orifice flow control device. A kink in the vapor hose can therefore result in escape of gasoline vapor in the vehicle tank fillpipe to the extent that the vapor recovery throughput is reduced at the flow control device.
In practice, the detection of a damaged vapor hose in the service station is not easy, since the outer hose might not show any physical sign of kinking. This type of defect can only be found through extensive testing or by a process of elimination of other vapor recovery related components.